A philosophical discussion of whether macroevolution is
reducible to microevolution, or if it stands as a separate
process in evolution

Whether or not there are barriers that prevent microevolution,
which creationists accept, from becoming macroevolution, which
they reject

Whether or not the idea of macroevolution can be falsified,
and whether specific accounts of macroevolution can be
falsified.

Antievolutionists argue against macroevolution so loudly that
some people think they invented the term in order to dismiss
evolution. But this is not true; scientists not only use the
terms, they have an elaborate set of models and ideas about it,
which of course antievolutionists gloss over or treat as being
somehow problems for evolutionary biology.

A later version will add a section on how creationists "move
the goalposts" when confronted with undeniable evidence of
macroevolution, but for now see the sister FAQ of Douglas Theobald.

The reader is invited to skip the section on reduction. This
is a largely philosophical discussion included because it is a
debate within the scientific community. It has no impact on the
fact of above-species evolution (that is, on
speciation, common descent, and pattern in the phylogenetic
record). But it is often the subject of heated debates in forums
discussing evolution in the context of creationism.

As Humpty Dumpty said to Alice:

There's glory for you!'
'I don't know what you mean by "glory,"' Alice said.
Humpty Dumpty smiled contemptuously. 'Of course you don't – till
I tell you. I meant "there's a nice knock-down argument for
you!"'
'But "glory" doesn't mean "a nice knock-down argument,"' Alice
objected.
'When I use a word,' Humpty Dumpty said in
rather a scornful tone, 'it means just what I choose it to mean
– neither more nor less.'
'The question is,' said Alice, 'whether you CAN make words mean
so many different things.'
'The question is,' said Humpty Dumpty, 'which is to be master –
that's all.'

Words are not the master of science; science is, or should be,
the master of its words. But we can inquire how scientists use
their words, and whether they use them consistently. And having
done that, we can inquire whether others who are not scientists read too much
into them, or use them in a totally different way.

First, we have to get the definitions right.
The following terms are defined: macroevolution, microevolution,
cladogenesis, anagenesis, punctuated equilibrium theory, phyletic
gradualism

Creationists often assert that "macroevolution" is not proven,
even if "microevolution" is, and by this they seem to mean that
whatever evolution is observed is microevolution, but the rest is
macroevolution. In making these claims they are misusing
authentic scientific terms; that is, they have a non-standard
definition, which they use to make science appear to be saying
something other than it is. Evolution proponents often say that
creationists invented the terms. This is false. Both
macroevolution and microevolution are
legitimate scientific terms, which have a history of changing
meanings that, in any case, fail to underpin creationism.

In science, macro at the beginning of a word just means "big",
and micro at the beginning of a word just means "small" (both
from the Greek words). For example, "macrofauna" means big
animals, observable by the naked eye, while "microfauna" means
small animals, which may be observable or may not without a
microscope. Something can be
"macro" by just being bigger, or there can be a transition that
makes it something quite distinct.

In evolutionary biology today, macroevolution
is used to refer to any evolutionary change at or above the
level of species. It means at least the splitting
of a species into two (speciation, or cladogenesis, from
the Greek meaning "the origin of a branch", see Fig. 1) or the
change of a species over time into another (anagenetic
speciation, not nowadays generally accepted [note 1]). Any changes that occur at higher levels,
such as the evolution of new families, phyla or genera, are
also therefore macroevolution, but the term is not
restricted to those higher levels. It often also means long-term
trends or biases in evolution of higher taxonomic levels.

Microevolution refers to any evolutionary
change below the level of species, and refers to changes
in the frequency within a population or a species of its alleles
(alternative genes) and their effects on the form, or phenotype,
of organisms that make up that population or species. It can also
apply to changes within species that are not genetic.

Figure 1: Anagenesis and cladogenesis.
In this example, species A anagenetically changes over time to become species B,
while species B cladogenetically changes over time by splitting
into species C and D, neither of which are very different from B
or each other. The anagenesis axis represents change of form,
either genetic or phenotypic. The cladogenetic axis represents
isolation of species from each other (for example, reproductive
isolation). Of course, cladogenesis and anagenesis can often go
hand-in-hand as well. Anagenesis is not regarded by most scientists as "real" speciation, although it is indistinguishable in the fossil record from a cladogenetic event.

Another way to state the difference is that macroevolution is
between-species evolution and microevolution is within-species
evolution. Sometimes, macroevolution is called "supraspecific
evolution" (Rensch 1959, see Hennig 1966: 223-225).

There are various views of the dynamics of macroevolution.
Punctuated Equilibria are patterns
of change that indicate stasis, or long periods of time where species
exhibit very little change. There are several hypotheses that attempt to
explain stasis. The current consensus among paleontologists is that
large populations are buffered against evolutionary change by natural
selection or genetic drift. Evolutionary change becomes easier when
populations split into smaller demes. This change can be "locked in" if
the subpopulations evolve reproductive isolation and become separate
species. That's why change is associated with cladogenesis.
Phyletic gradualism suggests that species continue to adapt to
new challenges over the course of their history (see Fig. 1). Species
selection and species sorting theories think that there are
macroevolutionary processes going on that make it more or less likely
that certain species will exist for very long before becoming extinct,
in a kind of parallel to what happens to genes in microevolution.

How did the terms enter into scientific use,
and what has happened to them since?

In the "modern synthesis" of neo-Darwinism, which developed in
the period from 1930 to 1950 with the reconciliation of evolution
by natural selection and modern genetics, macroevolution is
thought to be the combined effects of microevolutionary
processes.

The terms macroevolution and microevolution
were first coined in 1927 by the Russian entomologist Iuri'i
Filipchenko (or Philipchenko, depending on the transliteration),
in his German-language work Variabilität und Variation, which was an early
attempt to reconcile Mendelian genetics and evolution.
Filipchenko was an evolutionist, but as he wrote during the
period when Mendelism seemed to have made Darwinism redundant,
the so-called "eclipse of Darwinism" (Bowler 1983), he was not a Darwinian, but an
orthogeneticist (he believed evolution had a direction).
Moreover, Russian biologists of the period had a history of
rejecting Darwin's Malthusian mechanism of evolution by
competition (Todes 1989).

In Dobzhansky's founding work of the Modern Synthesis,
Genetics and the Origin of Species, he began by
saying that "we are compelled at the present level of knowledge
reluctantly to put a sign of equality between the mechanisms of
macro- and microevolution" (1937: 12), thereby introducing the
terms into the English-speaking biological community (Alexandrov 1994). Dobzhansky had been
Filipchenko's student and regarded him as his mentor. In science
as in all academic disciplines, it is difficult to deny a major
tenet of one's teachers due to filial loyalty, and Dobzhansky,
who effectively started the modern Darwinian synthesis with this
book, found it disagreeable to have to deny his teacher's views
(Burian 1994).

The term fell into limited disfavour when it was taken over by
such writers as the geneticist Richard Goldschmidt (1940) and the paleontologist
Otto Schindewolf to describe their orthogenetic theories. As a
result, apart from Dobzhansky, Bernhardt Rensch and Ernst Mayr,
very few neo-Darwinian writers used the term, preferring instead
to talk of evolution as changes in allele frequencies without
mention of the level of the changes (above species level or
below). Those who did were generally working within the
continental European traditions (as Dobzhansky, Mayr, Rensch,
Goldschmidt, and Schindewolf are) and those who didn't were
generally working within the Anglo-American tradition (such as
John Maynard Smith and Richard Dawkins). Hence, use of the term
"macroevolution" is sometimes wrongly used as a litmus test of
whether the writer is "properly" neo-Darwinian or not (Eldredge 1995: 126-127).

The term was revived by a number of mainly paleontological
authors such as Steven Stanley (1979),
Stephen Jay Gould and Niles Eldredge, the authors of punctuated
equilibrium theory (see Eldredge
1995), who argued that something other than within-species
processes are causing macroevolution, although they disavow the
view that evolution is progressive. Many paleontologists have
held that what happens in evolution beyond the species level is
due to processes that operate beyond the level of populations –
for example, the notion of species selection (the idea that
species themselves get selected similarly to the way alleles get
selected within populations, see Grantham
1995, Rice 1995, and Stidd and Wade 1995 for reviews and
discussions).

The idea that the origin of higher taxa such as genera
requires something special is often based on the misunderstanding
of the way in which new lineages arise. The two species that are
the origin of canine and feline lineages probably differed very
little from their common ancestral species and each other. But
once they were taxonomically isolated from each other, they
evolved more and more differences that they shared internally but
that other lineages didn't. This is true of all lineages back to
the first eukaryotic (nuclear) cell. Even the changes in the
Cambrian explosion are of this kind, although some (e.g.,
Gould 1989) think that the genomes (gene
structures) of these early animals were not as tightly regulated
as modern animals, and therefore had more freedom to change.

Ways in which the term "macroevolution" is
used by scientists. Some are exact in the way they use it, while
others are less exact. These usages are not all the same, and
this causes some confusion. Why do scientists not agree on the
meaning of their terms?

The meaning modern authors give to the terms "macroevolution"
and "microevolution" is often confusing, and varies according to
what it is they are discussing. This is particularly the case
when "large-scale" evolutionary processes are being discussed.
For example, R. L. Carroll, in his undergraduate textbook
(1997: 10) defines microevolution as
"involving phenomena at the level of populations and species" and
macroevolution as "evolutionary patterns expressed over millions
and hundreds of millions of years". Eldredge says,
"Macroevolution, however it is precisely defined, always connotes
"large-scale evolutionary change" (1989: vii) and throughout his
book speaks of macroevolution as roughly equivalent to the
evolution of taxa that are of a higher rank than species, such as
genera, orders, families and the like. In his book
Evolution, Mark Ridley defines the terms thus
(2004: 227):

Macroevolution means evolution on the grand scale, and it is
mainly studied in the fossil record. It is contrasted with
microevolution, the study of evolution over short time periods,
such as that of a human lifetime or less. Microevolution
therefore refers to changes in gene frequency within a population
.... Macroevolutionary events events are much more likely to take
millions of years. Macroevolution refers to things like the
trends in horse evolution ... or the origin of major groups, or
mass extinctions, or the Cambrian explosion .... Speciation is
the traditional dividing line between micro- and
macroevolution.

There are many papers published that use the term in this
"higher category" way; why is that?

Science is not always consistent in its use of terms; this is
the source of much confusion. Sometimes this is carelessness, and
sometimes this is because of the way in which terms are developed
over time. When biologists and paleontologists talk about
macroevolution in the sense of "large-scale" evolution, they are
strictly speaking meaning only a part of the phenomena the term
covers, but it is the most interesting part for those
specialists. That is, they are talking about the
patterns of well-above-species-level evolution (Smith 1994).

In order to have a pattern you have to be able to compare
three or more species (Fig. 2). On its own, species A forms no
patterns, and so long as the changes within it do not result in a
new species, evolution is microevolutionary. If a new species B
splits from A, then you have macroevolution, but no patterns. For
there to be a pattern, you need to be able to say that one
species is more closely related to another than a third is (in
this case, that A is closer to B than it is to C).

Figure 2: If only two species or higher taxa
are identified (red set) there is no pattern. If three or more
(blue set) are included, then you are able to say that one is
more closely related, evolutionarily speaking, to another than
the third – in this case A and B are more closely related to each
other than either is to C, which split off earlier than the A/B
split.

The sorts of patterns that people are interested in when
discussing macroevolution tend to involve very many species,
either as a single large group ("higher taxon") or individually.
This is why many authors use the term "macroevolution" to mean
"large-scale evolution". However, just like anagenetic
speciation, "large-scale" is an arbitrary and often subjective
term, and the objective meaning of macroevolution is evolution at
or above the level of species [note 2].
Hence, Carroll's "definition" is problematic, despite his
prominence in the field, and this sort of confusion is to be
avoided. A previous attempt by Simpson
(1944) to introduce "megaevolution" for large-scale changes
also failed to be accepted, in part because it was never entirely
clear when "macro" ended and "mega" started.

A more considered definition is Levinton's: "I define the
process of macroevolution to be "the sum of those processes
that explain the character-state transitions that diagnose
evolutionary differences of major taxonomic rank" (Levinton 2001:2). Here, Levinton is trying to
define macroevolution in a way that is not prejudicial to the
debate he is writing about. It focuses on the characters of taxa,
and is neutral about what level of taxa are involved. He denies
the "species level" definition because he thinks, I believe
unnecessarily, that it makes macroevolution the study of
speciation. If the "pattern" analysis above is right, then
macroevolution only includes the study of speciation,
but it is hardly restricted to it. The scope of macroevolution
rises very far above that level. It's worth observing, though,
that Linnaean higher taxonomic levels are artificial, constructed
for convenience by systematists. Conclusions about evolution that
rely upon taxonomic levels like genera or families (e.g., Raup's
and Sepkoski's work on extinction, Raup and Sepkoski 1986, Sepkoski 1987, Raup
1991) must be taken with a grain of salt, since the taxon
levels are not the "same" across phylogenetically distant groups,
because they are not "natural", although they may, in fact (to be
shown), be good surrogates for phylogenetic diversity.

Incidentally, the study of speciation has taken off
significantly in recent years with some solid theoretical work
that suggests many macroevolutionary effects are indeed the
result of population level processes (Gavrilets 2003,
2004, Gavrilets and Gravner 1997). Using
the metaphor of the adaptive landscape – the field of all possible
gene recombinations for a population, each of which has a fitness
value assigned to it by the environment, Gavrilets and colleagues
have shown that what happens at the population level can indeed
lead to divergence between them, but that most of the time
populations that are maintained at a high fitness by selection
can nonetheless "drift" apart at random.

Hot news: philosophers of science like to
argue about the reduction of one kind of science to another. Many
have asked whether macroevolution reduces to microevolution. That
is, whether or not larger changes in evolution are "just the sum
of" small changes. We need to understand what "reduction" means
in the philosophy of science before we can start accusing people
of being "reductionists" or "holists".

From a philosophical perspective, one might say macroevolution
is just a bunch of microevolution. It's also just a bunch of
chemistry. And physics. These are unhelpful answers, so we might
find it worthwhile to ask how scientific domains relate to each
other. Whenever a scientist or philosopher asks if two theories
are reducible one to the other, there are several answers that
can be given. One is if the first theory being reduced A
is adequately captured by the reducing theory B. Another
is that A is not entirely captured by B. A third is
that A and B each have overlapping areas, and areas
only they capture. This is called the problem of theory
reduction.

Reduction has been a philosophical problem with respect to
science for about 60 years. It comes in three main varieties:
methodological reduction, which is the notion that one
ought to try to explain wholes in terms of the parts and
their interactions; ontological reduction, which is the
notion that all the units or entities of one theory are
composed of units or entities of another; and
metaphysical reduction, which is the claim that only one
kind of thing exists (also called "monism"). Ontological
reduction includes reducing all the laws and dynamic
generalisations of the A theory to laws and dynamic
generalisations of the B theory. In philosophy of science,
the case is often put in just these terms, but increasingly
philosophers are attending to the objects of scientific theories
as well as the models.

Consider atoms, as an example. At the time Dalton proposed
atoms, he was trying to explain larger things in terms of smaller
things with properties that added up to the properties of the
whole. He did this because he felt it was a good rule to follow,
explaining wholes in terms of parts. So he was a
methodological reductionist, explaining things in terms
of ontological reduction. He wasn't a metaphysical reductionist,
though, if he allowed that reality comprised stuff other than
atoms – such as gravity or light (or God). A parallel case is
genetic reductionism, in which behaviours are "reduced"
to genes – it is both methodologically and ontologically
reductionist in the domain of behaviour and biology. It doesn't
assert that everything in biology is genetic, though, because we
know that how genes are expressed is affected by non-genetic
factors, such as the availability of food during crucial phases
of development.

The reductive relation between microevolution and
macroevolution is hotly debated. There are those who, with
Dobzhansky, say that macroevolution reduces to microevolution. We
can break this down to three claims: within the "universe" of
biology, one might say that everything biological is best
explained by microevolution (methodological), or that all
entities and processes of macroevolution are microevolutionary
(usually genetic – this is ontological), or that everything that
happens (in biology) is genetic (metaphysical). In the
metaphysical case, genes acquire an almost mystical significance,
and no serious biologist makes this claim, although opponents
accuse some (particularly Dawkins) of doing so.

The two reductive claims we will consider now are the
methodological and the ontological.

The methodological claim that macroevolution
(Ma) reduces to microevolution (Mi) is a claim that
the optimal solution for investigating evolution is to apply
modelling and testing by genetic techniques. And this has been
very successful. However, it has not been an unqualified success
– developmental biology is not easily reducible to
genetics, nor is ecology. Cell division, specialisation and
signalling explain development, and the relationship between
genes and these processes is equivocal – that is, some genes play
a role in many developmental processes, and many genes play a
role in pretty well all processes. Moreover, there are many other
things involved in development: epigenetic factors (para-genetic
inheritance and environmental modulation of genetic effects),
cytological inheritance (organelles, cell membranes, ribosomes
and enzymes from parent cells, and parent organisms). So genes on
their own are not enough to explain why evolution occurs along
the pathways that it has. One reaction to methodological
reductionism in biology has been to assert that genes are merely
"bookkeeping" entities for evolutionary investigation (Gould 2002). The methodological reduction is not
sufficient, even if genes turn out to be the only significant
"players" in evolution.

It is this assumption that antireductionists challenge in the
ontological reductionist case. There are entities and
processes, they say, that affect macroevolutionary dynamics which
are not in their nature microevolutionary. What could these
be?

Well, a list that reductionists would accept includes climate
change, geomorphological processes like mountain building,
tectonic isolation and drift, vulcanism, extraterrestrial
influences like bolide impacts, galactic wobble, precession of
the earth's axial rotation, and possibly even local stars
approaching and changing the impact on the earth of comets and
other bolides in a cycle averaging around 13 million years. The
point the reductionists would make, though, is that everything
that these things affect is microevolutionary – only the
frequencies of genes in populations, and so on. They serve as the
environment in which genes change their frequency (or fail to,
and the species goes extinct). What the "player" is in
microevolution is the population, comprising organisms, traits
and genes; in short, the gene pool. Nothing else is
important.

Nonreductionists will argue, however, that there are emergent
processes and entities in macroevolution that cannot be captured
ontologically. There are several candidates for these, each
challenged by reductionists. The basis for this is a view of
evolution as a series of inclusive hierarchical levels, each of
which is somewhat independent of the lower levels.

Figure 3. The
hierarchical relations between Macroevolution (Ma) and
Microevolution (Mi), and the Environment
(E). Mi consists of Organisms (O) and
their Interactions (I) together with factors from the
Environment. Examples A-G illustrate the levels of environmental
influence:

Mutation caused by chemical, thermal or radioactive
interference.

Heat shock on developing zygotes.

Local adaptation to a niche.

Climatological change causing migration.

Geographical isolation.

Environmental changes that cannot be adapted to for
historical or developmental reasons (causing
extinction).

Changes that affect speciation rates and type.

One thing to bear in mind is that in classical reductionism,
the arrow of causal direction is from the microlevel to the
macrolevel. But speciation and higher evolutionary processes
affect what happens at lower levels too. David Sepkoski has a
table that illustrates the kind of causal arrows that work
downwards as well as upwards:

The frequencies of objects like species or organisms or
alleles are affected by the contexts in which they occur, which
are usually processes at the next level up. His notation clearly
describes what may be called "Darwinian" in this respect,
although one might not necessarily accept that the
Macro2 level is "non-Darwinian", if by that is meant
something contrary to Darwinian evolution in a broader sense.

Species
selection/sorting

Elisabeth Vrba (Vrba 1985,
Gould and Vrba 1993) proposed that species come into being
and go extinct in a biased way. Generalist species
(eurytopes) tend to survive longer – when one food source
is unavailable, they switch to another until it comes back, thus
avoiding predator-prey cycles known as Lotka-Volterra cycles
(such as fox numbers dropping dramatically when rabbits are
over-predated). Specialist species (stenotopes), though,
are sensitive to the contingent changes forced by climate
changes – even long droughts. But specialists tend to speciate
more frequently, even if they go extinct more frequently, too, as
they adapt to loss of degradation of their food resources.
Selection is a process of differential survival correlated with
ecological success, so proponents of selection of this kind
consider this to be a selection process on species. Others refer
to it as a species "sorting" process (see Grantham 1995 for a review) because species
are not sufficiently like organisms/individuals. Gould published
an extensive discussion shortly before his death (Gould 2002: 644-673). It is worth noting here
that if species are selected, it is more like asexual evolution
than the evolution of sexual organisms, as species rarely evolve
by recombining lineages, or at least animal species don't. Plant
species often do (at about 5-10% of new species), and we have
insufficient evidence about other groups to generalise.

Some "Non-Synthesis" or post-Synthesis evolutionists think
that the processes that cause speciation are of a different kind
to those that occur within species. That is, they admit that
macroevolution occurs, but think that normal genetic change is
restricted by such proposed mechanisms as developmental
constraints. This view is originally associated with the names of
Schmalhausen and Waddington, who were often characterised as
being non-Darwinians by the modern synthesis theorists. However,
with the recent rise of the field known as "evo-devo", or
evolutionary developmental biology, many of the ideas proposed by
Waddington and others have been revisited (Schlichting and Pigliucci 1998,
Amundson 2005, Levinton 2001).

There are several kinds of constraints upon evolution. The
best known is of course selective constraints: some forms are
just not viable, one way or another. But developmental
constraints have been proposed to explain why, for example,
in centipedes the segment number is always an odd number
(Arthur 2003). In these cases, the
constraint is the nature of the developmental system itself.
Others (Schlichting and
Pigliucci 1998) consider this as much a case of selection as
anything else; the developmental system – indeed, the ability to
evolve – is subjected to selection as well. Historical
constraints form a kind of "you can't get there from here"
class. Once something has evolved, any state that requires
reversing the evolution of that trait to get somewhere else is
vanishingly unlikely. So the dynamics of the evolution of that
trait are constrained by what has evolved already.

The notion of a bauplan – a
German word meaning "blueprint" or "builder's plan" – has been
applied to evolution most notably by Gould and Lewontin (1979). Bauplans (the word takes the
English plural in this context) are the body plans of
phyla, the second highest Linnaean taxonomic level.
Since Georges Cuvier named them in the early 19th century, phyla
(singular phylum) have been seen as distinct and natural
groupings within animals (arguably not in plants, where the level
is Division). Bauplans have been tied into the notion of a
developmental and a historical constraint. There have been
criticisms of the notion of a bauplan as being mystical in its
causal power. Others see it as something that cannot be easily
modified by the processes of within-species (Mi)
evolution.

One of the claims made by nonreductionists is that evolution
occurs on emergent properties. An emergent property is one in
which the property of a higher level system or object cannot be
reduced to the properties of its constituent elements, but
instead it "emerges" from the interactions between them (O'Connor
and Wong 2002, Mandik
2004). Emergent properties were first proposed by,
coincidentally, a friend of Darwin's, G. H. Lewes, in the field
of psychology, but the idea goes back to J. S. Mill in 1843. It
is often sloganized as "the whole is more than the sum of its
parts". Emergence was made an issue when applied, ironically
enough, to evolution in the 1920s by Jan Smuts and C. D.
Broad.

In evolution, a species is considered by some nonreductionists
as being a system that has properties above the level of the
individual, the kin, or the deme (breeding population), based
somewhat on Mayr's definition of a species as being a protected
breeding gene-pool (
Mayr 1996). This has been challenged on various grounds, not
least being that usually species appear to have no systematic
interactions between all its parts, and that the appropriate
level is the population.

In this writer's opinion, an emergent property is simply a
property that we have trouble
computing or predicting from a knowledge of the constituent
parts, but this simple dismissal is insufficient. We have to
identify the following aspects of the matter:

E: The environmental factors in which a species
exists – for example, geological and climatological changes

O: the properties of the organisms – for example
their traits and capacities severally

I: the interactions between the organisms
– for example, the lineages of heredity at the gene,
haplotype, genome and developmental levels of organization. Also,
the issue of organisms changing their environment through a
feedback process known as "niche construction" affects both
E and I (Oyama et al. 2000). We can set up the
reductionist position like this:

Reductionism: I(O
& E) → Ma

Ma is therefore the result of the union, in some way,
of E, O and I. This can
be massively complex and give rise to "sudden" changes [note 3], or hold the evolutionary process in a state
of stasis for long periods. Whether or not one wants to call this
"repeated rounds of microevolution" or not (Erwin 2000) is open to debate. And even if it
is, we still need to know the models for how they relate, and
what Mi covers.

The alternative, nonreductionism, posits that there are
properties and processes going on that cannot be reduced to
E, O, and Ialone. There
are some other things happening, call them M, that
need to be added into the mix.

Nonreductionism: M &
I(O & E) →
Ma

The arguments in biology are therefore concerned with what the
set of Ms might be, and how they operate.

It is a common claim of antievolutionists that there is a
limit to the amount of change that can be made. Creationists like
Gish (1979) claim that there is some
limitation within "basic kinds", without being able to express
exactly what basic kinds might be, or why change is restricted
within them. Others such as Johnson
(1991:18) claim that the limit lies in the availability of
genetic variety, and that when that limit is reached change
ceases, and although he does accept that "Darwinists" have "some
points to make", he is hardly fair when he says that variation
"might conceivably be renewed by mutation, but whether (and how
often) this happens is not known" (p19). Of course it
is known. We have had experimental evidence of rates of
mutations since the 1910s, and modern research both
mathematically and empirically confirms that rates of mutation
occur at around 0.1-1.5 per zygote, which is to say every embryo
has between 1/10th and 1.5 mutations on average, depending on
species (Crow 1997). The average mutation
rate – that is the average rate of persisting mutations in a
population – is 2.2 x 10-9 (Kumar and Subramanian 2002). Further,
genes do not have evolutionary histories that match exactly the
history of the species in which they exist; a field known as
coalescence genetics covers the ability of novel genes
to persist across speciation events, so that the variability is
"available" when it is selectively advantageous (Hey and Wakeley 1997). Note that this is
not to say that variation is maintained in order to be
available. It's just that it is available when selective
pressures change some of the time.

Creationists often say that species cannot be evolved from
each other because chromosome numbers are different. Humans, for
example, have 46 chromosomes, while chimpanzees have 48. But the
human chromosome 2 is the result of what is called a
Robertsonian fusion – the ancestral ape chromosomes 2p
and 2q appear to have fused at their ends (telomeres) to form the
human chromosome 2 (Williams, not
dated), and other species that have large chromosomal
differences can still interbreed (Nevo
et al. 1994). DNA aligns according to local sequence
rather than large-scale chromosome structure, and this is why
inversions and translocation in parts of the sequence still allow
interbreeding.

There appears to be no single amount of genetic variation
common between closely related species that prevents
interbreeding. In some, only a few are sufficient. In others,
much variation, such as the large chromosomal difference in
Nevo's mole rats, fails to prevent interbreeding.
Introgression, or the leakage of genes across species
boundaries, has been observed in lizards, plants, birds, and
fish.

In summary, there is no barrier to species forming. This may
not be enough to show that large-scale macroevolution
occurs, though, according to writers like Johnson and Hitching (1982), but the logic here implies
some causal force actively preventing change, rather
than a problem with change occurring. For if there is enough
change to form new species, and each species is slightly
different from its ancestor, then simple addition shows that many
speciation events can cause large-scale evolution over enough
time. A journey of a thousand miles begins with a single step.
Conversely, many single steps can traverse long distances. There
is no evidence of any kind of barriers to large-scale change
(Brauer and Brumbaugh 2001),
although creationists are free to offer some.

Antievolutionists try to make out that macroevolution is a
tautology, the way they claim that natural selection is a
tautology. The implication is that macroevolution cannot be
tested and shown to be wrong, and therefore it is not
science.

To clarify this, consider what it is that scientists test when
they test a hypothesis. Let's suppose that we are testing the
idea that global warming is caused by a rise in CO2 in
the atmosphere. There are two parts to this – one claim is that
CO­2 causes the retention of solar and other heat,
and the second is that this has happened in the past and is
actually happening now. If you show that in a particular case
global warming didn't happen (say, in the period of the last
interglacial), you haven't thereby shown that CO2
doesn't cause global warming, nor that it isn't doing so
now. All you have tested is a particular case.

We can test a particular claim of macroevolution. We
can test, for example, if weasels are more closely related to red
pandas than bears are (Flynn and
Nedbal 1998, Flynn et al.
2000). This is a test of a particular evolutionary
tree or scenario. It tests a historical reconstruction.
If shown, on the basis of the evidence and the best data, to be
wrong, then that history has indeed been falsified. But can we
test the idea of common descent? It is not possible to
show that something never occurred, but it is very easy to show
that where it ought to occur, it either has or it hasn't. Science
will not retain a bad idea when it is shown repeatedly not to
explain what we have a right to expect it to explain (this is one
reason why creationism was dropped from science back in the
1850s). If macroevolution persistently were shown to run counter
to the data, then science would drop it and look for another
solution.

Moreover, science has to an extent falsified the
initial conception of macroevolution. The original idea was that
evolution formed only tree-like patterns – species split like
branches. A growing consensus has argued that both hybridisation
(species recombining) and lateral genetic transfer (genes
crossing the taxonomic boundaries individually or as part of
symbiotic organisms that are taken into the "host" taxon's
cellular machinery) are more common than we had previously
thought. Macroevolution of species is still regarded as the most
common way that the diversity of life has developed, but the
"tree" now has "vines" that hang across the branches of single
celled organisms (Fig. 4).

Figure 4. Evolutionary
"vines" Lateral genetic transfer across the tree of life. Taken
from Carl Zimmer's blog,The
Loom, based on work done by Victor
Kunin, et al. This image covers only bacteria and archaea,
but the same conclusions apply at the wider scale of other single
celled organisms. [Full-sized
image]

So the Common Descent Hypothesisas we might call the
general idea, or the notion of Descent with modification
as Darwin called it in his correspondence, is tested every time a
particular hypothesis is tested. When there are problems
in enough phylogenies, then Common Descent may be rejected. So
far, though, it is a very good first approximation, and the fact
that revisions can be and have been made show that it is neither
dogma, nor insulated from data.

Is Microevolution distinct from Macroevolution and vice versa?
We concluded that this depends very much on what is meant by
"distinct" and so forth. All phenomena of microevolution –
evolution below the species level – must
necessarily have some effect above the species level.
But whether this is an additive effect or not depends on the
complexity of the relationships between the two levels in each
case. At least some macroevolution is the result of
microevolutionary processes. So we are only asking now if
all is. This is open to debate: the E
(environmental) factors that affect macroevolution are not
within-species (Mi) forces, but do microevolutionary
processes like gene frequency changes necessarily mediate them?
And this question is still unresolved amongst specialists. One
thing we can say now, though, is that we cannot draw a
simple equals sign between the two domains. It is an
open question, one much argued within evolutionary biology and
related disciplines, whether Mi = Ma in any
sense.

Ontologically, all the objects of Ma are accounted for
by the objects of Mi plus the objects and processes of
E. However, we can't just assume all the
processes of Ma are just the aggregate sum of the
processes of Mi – this needs to be shown. Methodologically,
we can not predict the outcomes of Ma from a knowledge of
the states of Mi plus E. This is not because
the outcomes of Ma are not the result of Mi and so
on, necessarily, but because we cannot compute in a reasonable
time those outcomes – too many variables, conditions, and
interconnections (Dupré 1993,
Rosenberg 1994).

But this doesn't mean that we can say that it is impossible to
evolve from one group to another because there is a barrier, as
creationists claim. Genes and developmental sequences are
extremely modifiable, and to date no barrier has been found, nor
any reason to suspect one exists. All modern biology accepts that
Ma is possible, through biological processes. The question
is, in what ways? And that is a matter for empirical
investigation, which is ongoing, and through which we are
learning new things.

Macroevolution is at least evolution at or
above the level of speciation, but it remains an open debate
among scientists whether or not it is solely the end product of
microevolutionary processes or there is some other set of
processes that causes higher level trends and patterns. It is
this writer's opinion that macroevolutionary processes are just
the vector sum of microevolutionary processes in conjunction with
large scale changes in geology and the environment, but this is
only one of several opinions held by specialists.

The misuse of the terms by creationists is all their own work.
It is not due to the ways scientists have used them. Basically
when creationists use "macroevolution" they mean "evolution which
we object to on theological grounds", and by "microevolution"
they mean "evolution we either cannot deny, or which is
acceptable on theological grounds".

This is not because species do
not change over time – some do. This is because there is no
objective and common criterion for when a species has changed
enough to count as a new species. The decision to count a species
as a new one when there is no splitting of the species into two
or more is a matter of personal taste or convention, which means
that such decisions tell us more about the preferences of the
scientist than they do about the organisms.

The definition of what counts
as a "species" is itself debated. Most often, for organisms that
reproduce sexually, it is the biological species concept, where
organisms are of the same species if they can reproduce together
and their progeny are fertile. However, this does not work in
asexual organisms, and often fails in organisms that can
interbreed across large taxonomic gaps. See The Index of Creation Claims CB801
for references.

Terms like "sudden" or "rapid"
or "slow" or "abrupt" are relative terms (Loon 1999). A lot of confusion occurs when people
used to thinking at one timescale, such as the geological, in
which an event that takes 5 million years can be "sudden", talk
to people for whom "sudden" means a few generations – which
depending on the organisms can be a couple of hours, months,
years or centuries. Similarly, "qualitative" change is relative
to the measures used. Something that is qualitatively different
on one measure can be a simple extrapolation on another (for
example, if exponential scales are used to graph the change). I
recommend removing such relative terms from this discussion
whenever possible, or quantifying them exactly, to avoid
misunderstanding.

Amundson, R.
2005. The changing role of the embryo in evolutionary
biology: structure and synthesis, Cambridge studies
in philosophy and biology. New York: Cambridge University
Press.
[Amazon]
[Powell's]
[Barnes and Noble]

Todes, D. P. 1989.
Darwin without Malthus: The Struggle for Existence in
Russian Evolutionary Thought, Monographs on the History and
Philosophy of Biology. New York: Oxford University
Press.
[Amazon]
[Powell's]
[Barnes and Noble]

Acknowledgements: Thanks to Larry Moran, Matt
Silberstein, Tom Scharle, Douglas Theobald, Pete Dunkelberg, Josh
Zelinsky, Chris Rohrer, Erik W., and many others on the
newsgroup, for comments, criticisms and outright disagreement.
Thanks also to Carl Zimmer for the "vines" figure, and to David
Sepkoski and Anya Plutynski for copies of their forthcoming articles.